Speed control circuit for a DC brushless motor

ABSTRACT

A motor speed control circuit includes a switch circuit and a voltage-detection circuit. The voltage-detection circuit is actuated depending upon high or low voltage of a power supply to thereby cause the switch circuit to output high or low operating voltage to a driver circuit of the motor in response to changes of the voltage of the power supply. The motor is adjusted and operated at desired speeds by a rated range of input voltages according to the high or low operating voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a speed control circuit for a dcbrushless motor and more particularly to a switch circuit and avoltage-detection circuit for changing speeds of the dc brushless motorin response to changes in voltage of the power supply.

2. Description of the Related Art

Currently, fans are used for forced air cooling of electronic devices,such as a power supply, CPU of a computer, display card, and computercomponents etc. recently, there is a trend to adjustably operate fans athigh or low speed depending upon ambient conditions. In high temperatureambiance, a fan must operate at high speed to reduce or remove heatproduced by electronic components so as to ensure that these electroniccomponents are operated normally and that the heat will not damage theelectronic components. In low temperature ambiance, the fan must operateat low adequate speed for saving power and decreasing noise.

FIG. 1 illustrates a schematic view of a traditional driver circuit fora single phase dc brushless motor.

Referring to FIG. 1, the driver circuit for the single phase dcbrushless motor includes a coil L1, a Hall element IC1, a driver elementIC2, and a capacitor C1. The Hall element IC1 is used for detectingchanges of a permanent magnetic field of a rotor to thereby outputweaker Hall voltage. And then the Hall voltage is magnified by thedriver circuit IC2 to further output alternative signals (squarewaveforms) from two terminals O1 and O2 to the bridging transistors Q1through Q4. The coil L1 is actuated in turn to change current indirection so as to rotate the rotor.

FIG. 2 illustrates a schematic view of a traditional driver circuit fora double phase dc brushless motor.

Referring to FIG. 2, the driver circuit for the double phase dcbrushless motor includes a first coil L1, a second coil L2, a capacitorC1, a Hall driver element IC3, a first resistor R1, a second resistorR2, and a transistor Q1. The resistors R1 and R2 and transistor Q1 forma rotational speed detective circuit. The Hall driver element IC3 isused for detecting changes of a permanent magnetic field of a rotor andthen the Hall voltage is magnified to output alternative signals (squarewaveforms) from two terminals O1 and O2 to the coils L1 and L2respectively. The coils L1 and L2 are actuated in turn to change currentso as to rotate the rotor.

In a cooling fan system, there is generally two voltage levels to adriver circuit for various desired amounts of forced air. For example, ahigh voltage level of 12V control a fan operating at 3000 rpm, and a lowvoltage level of 6V controls the fan operating at 1500 rpm. Therefore,the cooling fan system operates at different speeds to provide coolingcapacity depending upon the ambient condition.

FIG. 3 illustrates a voltage diagram for speed of a traditional dcbrushless motor. Between two specific voltages, ranging from 12V to 6Vfor example, are a high and a low speed. However, the voltage-to-speedcurve is nonlinear between the two voltages. When 12V is input to thecooling fan system, the motor is operated the high speed of 3000 rpm;but when the cooling fan system input is changed to 6V, the motor isoperated at a low speed of 2000 rpm as a result of the nonlinearity.Accordingly, the cooling fan system fails to certainly operate at apredetermined low speed of 1500 rpm to meet the system demand.

The motor structure and the nonlinear feature limit the traditional dcbrushless motor whose speed cannot be adjusted depending upon the systemneeds. In order to improve these limitations of the traditional motor toperform desired speeds, it is necessary to be redesigned.

The present invention intends to provide a speed control circuitcomprising a switch circuit and a voltage-detection circuit used foradjusting an operating voltage depending upon speeds of the motor. Theoperating voltage of desired speed of the motor is determined by therated range of input voltage in such a way to mitigate and overcome theabove problem.

SUMMARY OF THE INVENTION

The primary objective of this invention is to provide a speed controlcircuit for a dc brushless motor comprising a switch circuit and avoltage-detection circuit used for adjusting operating voltage dependingupon speeds of the motor.

The secondary objective of this invention is to provide a speed controlcircuit for a dc brushless motor using a switch circuit to determine anoperating voltage of low speed.

The present invention is a speed control circuit for a dc brushlessmotor. The speed control circuit includes a switch circuit and avoltage-detection circuit. The voltage-detection circuit is actuateddepending upon high or low voltage of power supply to thereby determinethe switch circuit to output high or low operating voltage to a drivercircuit of the motor in response to changes of the voltage of the powersupply. The motor is adjusted and operated at desired speeds by a ratedrange of input voltage according to the high or low operating voltage.

Other objectives, advantages and novel features of the invention willbecome more apparent from the following detailed description and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in detail with reference tothe accompanying drawings wherein:

FIG. 1 is a schematic view of a traditional driver circuit for a singlephase dc brushless motor;

FIG. 2 is a schematic view of a traditional driver circuit for a doublephase dc brushless motor;

FIG. 3 is a voltage diagram for speed of a traditional dc brushlessmotor;

FIG. 4 is a block diagram of a speed control circuit for a dc brushlessmotor in accordance with the present invention;

FIG. 5 is a voltage diagram for speed of a speed control circuit inaccordance with the present invention;

FIG. 6 is a schematic view of a speed control circuit in accordance witha first embodiment of the present invention applied in a driver circuitfor a single phase dc brushless motor; and

FIG. 7 is a schematic view of a speed control circuit in accordance witha second embodiment of the present invention applied in a driver circuitfor a double phase dc brushless motor.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, there are two embodiments of the presentinvention shown therein, which include generally a driver member and aspeed control member.

FIG. 4 illustrates a block diagram of a speed control circuit for a dcbrushless motor in accordance with the present invention.

Referring initially to FIG. 4, the speed control circuit 10 includes aswitch circuit 11 comprising an adjusting resistor and avoltage-detection circuit 12. The switch circuit 11 is adapted tocontrol power supply to output a high or low operating voltage to adriver circuit 1 of the motor. And the voltage-detection circuit 12 isadapted depending upon high or low voltage of the power supply toactuate the switch circuit 11 in response to changes of the voltages ofthe power supply.

FIG. 5 illustrates a voltage diagram for speed of a speed controlcircuit in accordance with the present invention. Referring to FIGS. 4and 5, when the system is input high voltage 12V, the switch circuit 11is actuated to be closed by the voltage-detection circuit 12 and thusthe motor can be input a high operating voltage 12V from the switchcircuit 11. The motor is capable of operating and maintaining at highspeed 3000 rpm until the input operating voltage is changed to a lowvoltage. When the system is changed to be input a low voltage 6V, theswitch circuit 11 is actuated to be opened by the voltage-detectioncircuit 12 and thus the motor can be input a predetermined low operatingvoltage xV dropped by the adjusting resistor. The motor is capable ofoperating and maintaining at low speed 1500 rpm.

FIG. 6 illustrates a speed control circuit in accordance with a firstembodiment of the present invention applied in a driver circuit for asingle phase dc brushless motor. Referring to FIGS. 1 and 6, referencenumerals of the first embodiment have applied the identical numerals ofthe traditional single phase dc brushless motor. The driver circuit ofthe first embodiment has the similar configuration and same function asthe traditional driver circuit and the detailed descriptions areomitted.

Referring now to FIG. 6, the speed control circuit 10 in accordance withthe first embodiment of the present invention is connected to the drivercircuit 1 of the single phase dc brushless motor. The speed controlcircuit 10 includes a switch circuit 11 consisted of a transistor Q2, aresistor R3, and a resistor R4. The resistor R4 is an adjusting resistorRx. When the transistor Q2 is cut off, the voltage of the power supplyVcc is shunted to the resistor R4 and thereby adjusted or dropped. Andthen this voltage drop is output to the driver circuit 1 of the motor.

Referring again to FIG. 6, the speed control circuit 10 further includesa voltage detection circuit 12 including a transistor Q3, a resistor R5,and a resistor R6. When the transistor Q3 is saturated, a current fromthe power supply Vcc can pass a base of the transistor Q2 and theresistor R3 to form a bias current between a collector and an emitter ofthe transistor Q3. Consequently, the transistor Q2 is saturated, and thepower supply Vcc is output to the driver circuit 1 through an emitterand a collector of the transistor Q2 so as to output a high operatingvoltage. This high operating voltage is input into the driver circuit 1and thus the motor (coil L1) is operated at high speed, as shown in FIG.5. When the transistor Q3 is cut off, the voltage of the collector ofthe transistor Q3 rises and cuts off a bias current between an emitterand abase of the transistor Q2. Consequently, the transistor Q2 (biascurrent) is cut off, and the power supply Vcc is input into the resistorR4 and dropped so as to output a low operating voltage. This lowoperating voltage is input into the driver circuit 1 and thus the motor(coil L1) is operated at low speed, as shown in FIG. 5. The resistors R5and R6 form a voltage divider. The voltage divider divides the voltageof the power supply Vcc to output a high voltage level (0.6 to 0.7V) tosaturate the transistor Q3 when the power supply has risen above areference voltage (xV). The transistor Q2 is actuated to saturate bymeans of switching to saturate the transistor Q3. By contrast, thevoltage divider output a low voltage level to cut off the transistor Q3when the power supply has fallen below the reference voltage. Similarly,the transistor Q2 is actuated to cut off by means of switching to cutoff the transistor Q3.

FIG. 7 illustrates a speed control circuit in accordance with a secondembodiment of the present invention applied in a driver circuit for adouble phase dc brushless motor. Referring to FIGS. 2 and 7, referencenumerals of the second embodiment have applied the identical numerals ofthe traditional double phase dc brushless motor. The driver circuit ofthe second embodiment has the similar configuration and same function asthe traditional driver circuit and the detailed descriptions areomitted.

Referring now to FIG. 7, the speed control circuit 10 in accordance withthe second embodiment of the present invention is connected to thedriver circuit 1 of the double phase dc brushless motor. The speedcontrol circuit 10 includes a switch circuit 11 consisted of atransistor Q2, and a resistor R4. The resistor R4 is an adjustingresistor Rx. When the transistor Q2 is cut off, the Hall driver circuitIC3 is dropped by the resistor R4 and then grounded. Thus the motor isoperated at low speed, as shown in FIG. 5. When the transistor Q2 issaturated, the Hall driver circuit IC3 is grounded through a collectorand an emitter of the transistor Q2. Thus the motor is operated at highspeed, as shown in FIG. 5.

Referring again to FIG. 7, the speed control circuit 10 further includesa voltage-detection circuit 12 consisted of a resistor R5, and aresistor R6. The resistors R5 and R6 consisted in a voltage divider. Thevoltage divider divides the voltage of the power supply Vcc to output ahigh voltage level (0.6-0.7V) to saturate the transistor Q2 when thepower supply is risen above a reference voltage (xV). Consequently, theHall driver circuit IC3 is grounded through the transistor Q2 and thecoils L1 and L2 are operated at the high operating voltage. By contrast,the voltage divider outputs a low voltage level to cut off thetransistor Q2 when the power supply is fallen below the referencevoltage. Consequently, the Hall driver circuit IC3 is grounded throughthe resistor R4 and the coils L1 and L2 are operated at the lowoperating voltage.

Although the invention has been described in detail with reference toits presently preferred embodiment, it will be understood by one ofordinary skill in the art that various modifications can be made withoutdeparting from the spirit and the scope of the invention, as set forthin the appended claims.

What is claimed is:
 1. A speed control circuit for a dc brushless motor,wherein said speed control circuit is applied to a single phase dcbrushless dc motor, comprising: a switch circuit to output a high or lowoperating voltage to a driver circuit of the motor in response to highand low voltages input from a power supply; and a voltage-detectioncircuit to control the switch circuit to switch between the high or lowoperating voltage in response to changes in the voltage of the powersupply; wherein when the voltage-detection circuit senses a high voltageinput from the power supply, the switch circuit is actuated to outputthe high operating voltage to the driver circuit and thus the motor isoperated at a high speed; wherein when the voltage-detection circuitsenses a low voltage input from the power supply, the switch circuit isactuated to output the low operating voltage to the driver circuit andthus the motor is operated at a low speed, the low operating voltagebeing adjusted by the switch circuit relative to the low voltage inputfrom the power supply.
 2. The speed control circuit for a dc brushlessmotor as defined in claim 1, wherein the switch circuit includes a firsttransistor, a first resistor, and a second resistor; thevoltage-detection circuit includes a second transistor; wherein when thesecond transistor is saturated resulting in saturation of the firsttransistor, the power supply is output to the driver circuit through thefirst transistor so as to output the high operating voltage to thedriver circuit and thus the motor is operated at the high speed; andwherein when the second transistor is cut off resulting in cutting offthe first transistor, a voltage of the power supply is reduced by thesecond resistor so as to output the adjusted low operating voltage tothe driver circuit and thus the motor is operated at the low speed. 3.The speed control circuit for a dc brushless motor as defined in claim2, wherein the voltage-detection circuit further includes a thirdresistor and a fourth resistor arranged to form a voltage divider;wherein when the power supply voltage has risen above a referencevoltage, the voltage divider divides the voltage of the power supply tooutput the high voltage level to saturate the second transistor andsubsequently saturate the first transistor; and wherein when the powersupply voltage has fallen below the reference voltage, the voltagedivider outputs predetermined low voltage level to cut off the secondtransistor and subsequently cut off the first transistor.
 4. A speedcontrol circuit for a dc brushless motor, wherein the speed controlcircuit is applied to a double phase dc brushless motor, comprising: aswitch circuit to output a high or low operating voltage to a drivercircuit of the motor in response to high and low voltages input from apower supply; and a voltage-detection circuit to control the switchcircuit to switch between the high or low operating voltage in responseto changes in the voltage of the power supply; wherein when thevoltage-detection circuit senses a high voltage input from the powersupply, the switch circuit is actuated to output the high operatingvoltage to the driver circuit and thus the motor is operated at a highspeed; wherein when the voltage-detection circuit senses a low voltageinput from the power supply, the switch circuit is actuated to outputthe low operating voltage to the driver circuit and thus the motor isoperated at a low speed, the low operating voltage being adjusted by theswitch circuit relative to the low voltage input from the power supply.5. The speed control circuit for a dc brushless motor as defined inclaim 4, wherein the switch circuit includes a transistor, a firstresistor, and a second resistor; the voltage-detection circuit includesa second resistor, and a third resistor; wherein when the transistor iscut off, a Hall driver circuit is dropped by the first resistor toground and the motor is operated at the low speed; and wherein when thetransistor is saturated, the Hall driver circuit is grounded through thetransistor and thus the motor is operated at the high speed.
 6. Thespeed control circuit for a dc brushless motor as defined in claim 5,wherein the second and third resistors form a voltage divider; whereinwhen the power supply voltage has risen above a reference voltage, thevoltage divider divides the voltage of the power supply to output a highvoltage level to saturate the transistor and consequently the Halldriver circuit is grounded; and wherein when the power voltage hasfallen below the reference voltage, the transistor is cut off andconsequently the Hall driver circuit is grounded through the firstresistor.
 7. The speed control circuit for a dc brushless motor asdefined in claim 1, wherein the speed control circuit further comprisesan adjusting resistor (Rx) to thereby drop the low voltage of the powersupply that can output a predetermined low operating voltage to thedriver circuit of the motor.